[PATCH] xfs: fix race while discarding buffers [V4]

[Ben Myers]

Hi Carlos,

On Fri, Aug 10, 2012 at 03:01:51PM -0300, Carlos Maiolino wrote:
> While xfs_buftarg_shrink() is freeing buffers from the dispose list (filled with
> buffers from lru list), there is a possibility to have xfs_buf_stale() racing
> with it, and removing buffers from dispose list before xfs_buftarg_shrink() does
> it.
> 
> This happens because xfs_buftarg_shrink() handle the dispose list without
> locking and the test condition in xfs_buf_stale() checks for the buffer being in
> *any* list:
> 
> if (!list_empty(&bp->b_lru)
			     )

That's cruel and unusual punishment.
 
> If the buffer happens to be on dispose list, this causes the buffer counter of
> lru list (btp->bt_lru_nr) to be decremented twice (once in xfs_buftarg_shrink()
> and another in xfs_buf_stale()) causing a wrong account usage of the lru list.
> 
> This may cause xfs_buftarg_shrink() to return a wrong value to the memory
> shrinker shrink_slab(), and such account error may also cause an underflowed
> value to be returned; since the counter is lower than the current number of
> items in the lru list, a decrement may happen when the counter is 0, causing
> an underflow on the counter.
> 
> The fix uses a new flag field (and a new buffer flag) to serialize buffer
> handling during the shrink process. The new flag field has been designed to use
> btp->bt_lru_lock/unlock instead of xfs_buf_lock/unlock mechanism.
> 
> dchinner, sandeen, aquini and aris also deserve credits for this.
>
> Signed-off-by: Carlos Maiolino <cmaiolino at redhat.com>
> ---
>  fs/xfs/xfs_buf.c |  5 ++++-
>  fs/xfs/xfs_buf.h | 41 ++++++++++++++++++++++++-----------------
>  2 files changed, 28 insertions(+), 18 deletions(-)
> 
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index d7a9dd7..933b793 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -96,6 +96,7 @@ xfs_buf_lru_add(
>  		atomic_inc(&bp->b_hold);
>  		list_add_tail(&bp->b_lru, &btp->bt_lru);
>  		btp->bt_lru_nr++;
> +		bp->b_lru_flags &= ~_XBF_LRU_DISPOSE;
>  	}
>  	spin_unlock(&btp->bt_lru_lock);
>  }
> @@ -154,7 +155,8 @@ xfs_buf_stale(
>  		struct xfs_buftarg *btp = bp->b_target;
>  
>  		spin_lock(&btp->bt_lru_lock);
> -		if (!list_empty(&bp->b_lru)) {
> +		if (!list_empty(&bp->b_lru) &&
> +		    !(bp->b_lru_flags & _XBF_LRU_DISPOSE)) {
>  			list_del_init(&bp->b_lru);
>  			btp->bt_lru_nr--;
>  			atomic_dec(&bp->b_hold);
> @@ -1501,6 +1503,7 @@ xfs_buftarg_shrink(
>  		 */
>  		list_move(&bp->b_lru, &dispose);
>  		btp->bt_lru_nr--;
> +		bp->b_lru_flags |= _XBF_LRU_DISPOSE;
>  	}
>  	spin_unlock(&btp->bt_lru_lock);
>  
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index d03b73b..7c0b6a0 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -38,27 +38,28 @@ typedef enum {
>  	XBRW_ZERO = 3,			/* Zero target memory */
>  } xfs_buf_rw_t;
>  
> -#define XBF_READ	(1 << 0) /* buffer intended for reading from device */
> -#define XBF_WRITE	(1 << 1) /* buffer intended for writing to device */
> -#define XBF_READ_AHEAD	(1 << 2) /* asynchronous read-ahead */
> -#define XBF_ASYNC	(1 << 4) /* initiator will not wait for completion */
> -#define XBF_DONE	(1 << 5) /* all pages in the buffer uptodate */
> -#define XBF_STALE	(1 << 6) /* buffer has been staled, do not find it */
> +#define XBF_READ	 (1 << 0) /* buffer intended for reading from device */
> +#define XBF_WRITE	 (1 << 1) /* buffer intended for writing to device */
> +#define XBF_READ_AHEAD	 (1 << 2) /* asynchronous read-ahead */
> +#define XBF_ASYNC	 (1 << 4) /* initiator will not wait for completion */
> +#define XBF_DONE	 (1 << 5) /* all pages in the buffer uptodate */
> +#define XBF_STALE	 (1 << 6) /* buffer has been staled, do not find it */
>  
>  /* I/O hints for the BIO layer */
> -#define XBF_SYNCIO	(1 << 10)/* treat this buffer as synchronous I/O */
> -#define XBF_FUA		(1 << 11)/* force cache write through mode */
> -#define XBF_FLUSH	(1 << 12)/* flush the disk cache before a write */
> +#define XBF_SYNCIO	 (1 << 10)/* treat this buffer as synchronous I/O */
> +#define XBF_FUA		 (1 << 11)/* force cache write through mode */
> +#define XBF_FLUSH	 (1 << 12)/* flush the disk cache before a write */
>  
>  /* flags used only as arguments to access routines */
> -#define XBF_TRYLOCK	(1 << 16)/* lock requested, but do not wait */
> -#define XBF_UNMAPPED	(1 << 17)/* do not map the buffer */
> +#define XBF_TRYLOCK	 (1 << 16)/* lock requested, but do not wait */
> +#define XBF_UNMAPPED	 (1 << 17)/* do not map the buffer */
>  
>  /* flags used only internally */
> -#define _XBF_PAGES	(1 << 20)/* backed by refcounted pages */
> -#define _XBF_KMEM	(1 << 21)/* backed by heap memory */
> -#define _XBF_DELWRI_Q	(1 << 22)/* buffer on a delwri queue */
> -#define _XBF_COMPOUND	(1 << 23)/* compound buffer */
> +#define _XBF_PAGES	 (1 << 20)/* backed by refcounted pages */
> +#define _XBF_KMEM	 (1 << 21)/* backed by heap memory */
> +#define _XBF_DELWRI_Q	 (1 << 22)/* buffer on a delwri queue */
> +#define _XBF_COMPOUND	 (1 << 23)/* compound buffer */
> +#define _XBF_LRU_DISPOSE (1 << 24)/* buffer being discarded */

It's nice to have them lined up like that.

>  
>  typedef unsigned int xfs_buf_flags_t;
>  
> @@ -72,12 +73,13 @@ typedef unsigned int xfs_buf_flags_t;
>  	{ XBF_SYNCIO,		"SYNCIO" }, \
>  	{ XBF_FUA,		"FUA" }, \
>  	{ XBF_FLUSH,		"FLUSH" }, \
> -	{ XBF_TRYLOCK,		"TRYLOCK" }, 	/* should never be set */\
> +	{ XBF_TRYLOCK,		"TRYLOCK" },	/* should never be set */\

...and you got rid of an extra space here.

>  	{ XBF_UNMAPPED,		"UNMAPPED" },	/* ditto */\
>  	{ _XBF_PAGES,		"PAGES" }, \
>  	{ _XBF_KMEM,		"KMEM" }, \
>  	{ _XBF_DELWRI_Q,	"DELWRI_Q" }, \
> -	{ _XBF_COMPOUND,	"COMPOUND" }
> +	{ _XBF_COMPOUND,	"COMPOUND" }, \
> +	{ _XBF_LRU_DISPOSE,	"LRU_DISPOSE" }
>  
>  typedef struct xfs_buftarg {
>  	dev_t			bt_dev;
> @@ -124,7 +126,12 @@ typedef struct xfs_buf {
>  	xfs_buf_flags_t		b_flags;	/* status flags */
>  	struct semaphore	b_sema;		/* semaphore for lockables */
>  
> +	/*
> +	 * concurrent access to b_lru and b_lru_flags are protected by
> +	 * bt_lru_lock and not by b_sema
> +	 */
>  	struct list_head	b_lru;		/* lru list */
> +	xfs_buf_flags_t		b_lru_flags;	/* internal lru status flags */
>  	wait_queue_head_t	b_waiters;	/* unpin waiters */
>  	struct list_head	b_list;
>  	struct xfs_perag	*b_pag;		/* contains rbtree root */

This looks pretty good to me.  Looks like you've been careful about the
locking.

What was the symptom that led to the discovery of this problem?

Reviewed-by: Ben Myers <bpm at sgi.com>

Regards,
Ben